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Calculation to study reactions and impurities in ionic liquids as well as the gas phase behavior and thermal stability

Subject Area Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Term from 2006 to 2014
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 29242968
 
Final Report Year 2013

Final Report Abstract

The pure liquid state was intensively studied in our group in the first two periods of the priority program. , Important methodological knowledge was collected and developments with respect to force fields and density functional theory were made. For example, the charge values can have a great influence on the calculated structural and dynamical properties which was studied in collaboration with the Müller-Plathe group. , Furthermore, the role of hydrogen bonds on the structure and dynamics of [C4C1im][Br] was modeled and compared to experimental results of the groups of Kärger, Kremer and Giernoth. Currently, we are collaborating with the group of Profs. Padua and Costa Gomez as well as with the group of Prof. Fernandez, (Universidad de Santiago de Compostela).on the development of forces fields for ionic liquid interfaces. It is well known that dispersion interactions play an important role in ionic liquids. , , , We extended our studies to the gas phase. , The main interactions in the gas phase between the ethylsulfate anion and the [C2C1im] cation are the electrostatic and hydrogen-bonding interactions. Contrary, for the bulk phase, aggregation of the side chains occurs, and the coordination between the imidazolium ring and the cation involves all the hydrogen atoms of the imidazolium ring. Furthermore, we completed our studies on water(impurities)-IL interactions performed on [C2C1im][OAc]/water mixtures. We observed how water disturbs the near ordering found in the neat IL and how it inhibits the carbene formation indicated in neat [C2C1im][OAc]. Using classical MD the liquid-gas interface of several imidazolium and pyrrolidinium ILs was investigated, showing a strong ordering of the ions in the interfacial region. For ionic liquids with large side chains, the chains tend to protrude towards the gas phase, but their density is low.xi We have investigated the physical absorption of CO2 in ILs in collaboration with PD Stark and Prof. Nyulaszi. The important interactions sites were detected and we pointed out that the cation also has a significant role in the absorption of CO2. Compared to CO2, the solute-solvent interactions with SO2 showed similar interactions, but due to the higher polarity and the larger size, they are generally stronger than those with CO2. Revealing the cation and anion effects in general we found the larger and less coordinating the anions, the bigger the competition with the cations for the on-top place around the imidazolium ring is. Fluorination of the cationic side chain promotes the on-top position. Furthermore, aggregation of side chains was observed in all ILs. The aggregation of side chains is decreased by large and weakly coordinating anions and is increased by the fluorination of the cation. We also performed an AIMD-based metadynamics study on a simple model Diels-Alder reaction in the three different solvents, namely dichloromethane, [C2C1im][Cl] and [C2C1im][OAc]. The reaction barrier is not strongly affected by the choice of the solvent. Apart from the aforementioned physical absorption of CO2 in ILs, the more intense chemical absorption has also been investigated in [C2C1im][OAc]. In this specific IL the acetate can promote a carbene formation process by abstracting a single proton from the cation. This carbene reacts with the CO2 molecule, forming imidazolium-carboxylate derivatives. We have shown, that this reaction is possible only because the cation-CO2 and cation-anion interactions weaken the strong association between the anion and the solute. These results put the investigation of solute-solvent interactions in ILs into a new perspective in general, since it suggests that investigating the single anion-solute or cation-solute interactions may be misleading, and a larger assembly is needed to be taken into account to obtain a chemically relevant picture. We currently investigate the cis/trans conformation of a model dipeptide in water and in [C2C1im][OAc] in collaboration with the Bordusa group. Two imidazolium based liquids were used as a model system for the investigation of ionic liquid mixtures in collaboration with PD Stark. The main focus was the investigation of the changes in the molecular structure. The model system shows strongly ideal behavior with respect to these properties. The results suggest that ionic liquids with similar constituents give ideal mixtures with regard to structural properties. Since ILs are purely composed of ions, they possess a completely different structure compared to other solvents, viz. molecular liquids. This special ion screening leads to specific properties. By providing higher preference in stabilizing ionic substances, which is called the “ionic liquid effect”, different reaction mechanisms can be found. This effect is clearly observable in case of the formation of carbenes in imidazolium acetate ILs, which is significantly suppressed in the liquid compared to the gas phase. However, we have observed that this suppression can be (partially) cancelled out by inserting neutral molecules into the IL solution, which provides defects in the ion screening, i.e., an inverse ionic liquid effect. Accordingly, we have proposed that the presence of CO2 itself may catalyze the carbene formation in imidazolium-acetate ILs, which allows its chemical absorption.

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